Engine systems may be configured with exhaust gas recirculation (EGR) systems via which at least a portion of the exhaust gas is recirculated to the engine intake. Various sensors may be coupled in the engine system to estimate the amount of EGR being delivered to the engine. These may include, for example, various temperature, pressure, oxygen, and humidity sensors coupled to the engine intake manifold and/or the exhaust manifold.
One example of a boosted engine system having an intake humidity sensor is shown by Surnilla et al. in US application 2012/0227714. Therein, the humidity sensor is used to infer an ambient humidity. An engine controller may then adjust one or more engine operating parameters, such as fuel injection, valve timing, EGR, etc., based on the estimated ambient humidity.
However, the inventors herein have identified potential issues with such an approach. As an example, during boosted engine operation, any opening of a compressor bypass valve (such as to mitigate or reduce compressor surge) can confound the humidity sensor output. Specifically, opening of the compressor bypass valve may create a (temporary) backflow of air from the charge air cooler outlet location to the compressor inlet location. The back-flowing air may temporarily raise the compressor inlet pressure for a short duration until the boost pressure is sufficiently reduced. The short duration of the increase in pressure at the compressor inlet can cause the air to flow back through an intake air filter to the atmosphere. The intake humidity sensor located between the air filter and the compressor inlet (or at the air filter) will see this back-flowing air. If low pressure EGR was already flowing when the compressor bypass valve was opened, the back-flowing air may have a higher water content (over and above the ambient humidity) due to the presence of EGR. As a result, ambient humidity may be over-estimated by the humidity sensor. As such, this may corrupt the adjustment of the various engine operating parameters while degrading engine performance.
In one example, the above issue may be at least partly addressed by a method for an engine comprising: in response to opening of a compressor bypass valve while EGR is enabled, adjusting an engine operating parameter based on an output of an intake humidity sensor read before the opening of the bypass valve. In this way, a more reliable ambient humidity estimate may be used for engine adjustments.
As an example, during un-boosted engine operating conditions, as well as selected boosted conditions where a compressor bypass valve is not open (such as when the engine is operating within a surge margin), a current output of an intake humidity sensor may be stored and used to estimate an ambient humidity. Further, the current output may be used to adjust one or more engine parameters, such as a spark timing (or spark correction), condensation modeling, an EGR flow rate, humidity sensor diagnostics, etc. Then, during boosted conditions where the compressor bypass valve is opened, such as when the engine is operating at or beyond a surge margin, the current output of the humidity sensor may not be used. Instead, an output of the sensor read immediately before the opening of the bypass valve may be stored and used to estimate the ambient humidity. Further, the various engine operating parameters may be adjusted based on the stored humidity sensor output. After a threshold duration has elapsed since the completion of the compressor bypass valve event (that is, after a threshold duration since the compressor bypass valve has been closed), the current output of the humidity sensor may be re-used to estimate ambient humidity and adjust engine operating parameters.
In this way, a false reading of the humidity sensor is avoided. Specifically, the output of a humidity sensor may not be used during conditions when the output may be corrupted. By disabling use of a current output of the intake humidity sensor during conditions when a compressor bypass valve is open and backflow is increased, humidity overestimation due to EGR flow can be reduced. By freezing the humidity sensor output from before (e.g., immediately before) the compressor bypass event, and using the frozen sensor output to adjust engine operation, a more accurate ambient humidity estimate can be used to for engine adjustments. By unfreezing the humidity sensor output after (e.g., immediately after) a threshold duration since the compressor bypass event has elapsed, engine functions that depend on humidity information are not affected.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.